In a sign of the strain the Canadian government is feeling over development of the tar sands, Environment Minister Peter Kent said on Wednesday that opposition legislators who campaigned in Washington against the idea were treacherous.

The right-of-center Conservative government was taken aback last week when the U.S. administration delayed approval of TransCanada Corp's proposed Keystone XL pipeline designed to carry crude from the vast Alberta oil sands to refineries in Texas.

The Conservatives strongly back the pipeline. Opponents say development tar sands oil is particularly dirty, producing high amounts of greenhouse gases. They also say the pipeline could cause an environmental catastrophe if it ruptured.

Much to Kent's anger, two members of Parliament from the opposition New Democrats went to Washington this week to argue the pipeline should not go ahead until Canada has come up with a better plan to combat climate change.

One of the opposition parties has taken the treacherous course of leaving the domestic debate and heading abroad to attack a legitimate Canadian resource which is being responsibly developed and regulated, Kent told reporters.

Neither the chief spokesman for Prime Minister Stephen Harper nor a spokeswoman for Kent responded to requests for comment.

In late September, two senior government officials said a few hundred people who protested peacefully in Ottawa against the pipeline were extremists. Alberta is one of the Conservatives' main power bases.

Speaking in the House of Commons on Wednesday, Natural Resources Minister Joe Oliver said Keystone XL would generate hundreds of thousands of jobs, trillions of dollars in economic activity and revenue.